Conventionally, a microphone unit that has the function of converting an input sound into an electrical signal and outputting it is applied to various types of sound input devices (for example, sound communication devices such as a mobile telephone and a transceiver, information processing systems, such as a sound authentication system, that utilize a technology for analyzing an input sound and a recording device). Such a microphone unit, for example, may be required to suppress background noise and receive only near-sound or may be required to receive not only near-sound but also far-sound.
As an example of a sound input device incorporating a microphone unit, a mobile telephone will be described below. When a mobile telephone is used to start a call, a user generally holds the mobile telephone, and brings his mouth close to a microphone portion and uses it. Hence, the microphone incorporated in the mobile telephone is generally required to have the function of suppressing background noise and receiving only near-sound (function as a close-talking microphone). As the microphone described above, for example, a differential microphone described in patent document 1 is suitable.
However, among mobile telephones today, there is a mobile telephone that has the hands-free function of making a call without holding of the telephone when, for example, driving a car, and a mobile telephone that has the function of video recording. When the mobile telephone utilizing the hands-free function is used, since the mouth of the user is present in a position away from the mobile telephone (for example, in a position 50 cm away), a microphone is required to have the function of receiving not only near-sound but also far-sound. In video recording, since it is necessary to record the atmosphere of the place where the recording is performed, the microphone is required to have the function of receiving not only near-sound but also far-sound.
In other words, in recent years, the mobile telephone has become multifunctional, and thus the microphone incorporated in the mobile telephone is required to have both the function of suppressing background noise and receiving only near-sound and the function of receiving not only near-sound but also far-sound. One way to meet such a requirement is to separately incorporate, in a mobile telephone, a microphone unit having a function as a close-talking microphone and an omnidirectional microphone unit that can also receive far-sound.
Another way is to apply a microphone unit disclosed in, for example, patent document 2 to a mobile telephone. In the microphone unit disclosed in patent document 2, one of the two opening portions for inputting sound can be switched by an opening/shutting system between an open state and a closed state. When the two opening portions are open, the microphone unit disclosed in patent document 2 functions as a bidirectional differential microphone whereas, when one of the two opening portions is closed, it functions as an omnidirectional microphone.
When the microphone unit functions as a bidirectional differential microphone, since it is possible to suppress background noise and receive only near-sound, it is suitable for a case where the user uses the mobile telephone while holding it. On the other hand, when the microphone unit functions as an omnidirectional microphone, since it is also possible to receive far-sound, it is suitable for a case where the hands-free function or the video recording function is used.